Research Institute, Chiba Institute of Technology/Ocean Research Institute, University of Tokyo/Mineralogical Institute, Graduate School of Science, University of Tokyo/Mineralogical Institute, Graduate School of Science, University of Tokyo
抄録(英)
Mineralogical study of three apparently crystalline eucrites, Asuka (A)-87272,A-881388 and A-881394 revealed that their textures are not primary crystallization products from a magma. A-87272 is a monomict breccia, but the finegrained matrix is recrystallized to a granulitic texture with fine, rounded pyroxene crystals set in a plagioclase matrix. Large fragments of pyroxene are inverted to orthopyroxene with coarse exsolution lamellae on (001) and fine ones on (100). A-881388 contains a large, rounded opaque grain with a tail, and with an ilmenite-chromite-troilite-metal assemblage in fine-grained granulitic silicates, suggesting recrystallization. A-881394 is coarser grained than A-881388 and contains more magnesian pyroxene as in cumulate eucrites, but the plagioclase composition is extremely calcic (An 98) and the grains are composed with a few rounded grains. The A-881394 chromite, showing a pokilitic texture with rounded plagioclase and minor pyroxene, suggests a metamorphic texture. Our present interpretation for a common formation process among these eucrites is that despite their crystalline texture, they might have experienced extensive metamorphism after the initial crystallization in the early history of the crustal evolution.